Logic circuits

Which bit pattern represents −1 in 4-bit two's complement?

  1. A1111
  2. B1001
  3. C0001
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Answer: A. 1111

In two's complement, a negative number is obtained by inverting the bits of the positive number and adding 1. Inverting +1 (0001) gives 1110, and adding 1 gives 1111. Four-bit two's complement can represent −8 to +7. 1001 is −1 in sign-magnitude representation.

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